1. Technical Field
The embodiments herein generally relate to medical devices and assemblies, and more particularly to an orthopedic spring-loaded dynamic pedicle screw assembly used for surgical lumbar, thoracic, and cervical spine treatment.
2. Description of the Related Art
Surgical procedures treating spinal injuries are one of the most complex and challenging surgeries for both the patient and the surgeon. A common problem with the internal spinal fusion is how to secure the fixation devices without damaging the spinal cord. The pedicles are a favored area of attachment since they offer an area that is strong enough to hold the fixation device even when the patient suffers from osteoporosis (e.g., a disease of bone leading to an increased risk of fracture).
More recently, methods of internal fixation have utilized wires that extend through the spinal canal and hold a rod against lamina or that utilize pedicle screws, which extend into the pedicle and secure a plate, which extends across several vertebral segments. However, the wired implants include an increased risk of damage to the neural elements (e.g., spinal cord and nerve roots). On the other hand, the use of plates with the screws rigidly linked results in the direct transfer of loads at the bone screw interface, which is the weakest link in the fixation spine construction. This can result in breakage of the screw or failure of the bone screw interface prior to achieving fusion. In addition, the plate designs are generally bulky and tend to leave little surface for bone grafting and they typically cannot be easily contoured to account for the lateral curvature of the spine.
Additionally, some methods have used polyaxial screw systems for fixation. Conventional polyaxial screw systems typically consist of a screw-receiving portion with the bottom portion of the screw-receiving portion pivoting inside a bone screw, and a receiving rod. This typical conventional design necessitates the screw-receiving portion to have a narrow neck just above the entrance to the screw. This smaller and weaker neck portion is significantly further away from the forces being applied through the rod, which consequently allows a bigger moment arm and increases the chance of breakage at the weak neck portion.
Furthermore, most conventional screw assemblies generally do not allow for a bone screw to be tensioned in a given zone or range of angulations in the screw head. Consequentially, most conventional solutions use the rod portion of the system to provide the dynamism. Generally, most artificial discs currently being marketed do not typically offer any resistance at the extreme ranges of motion, and others tend to offer a “dead stop” that may cause implant failure or implant dislodging. As such, most surgeons would concede that the removal of a failed artificial disc is an extremely undesirable event that is fraught with major complications.
Moreover, when there are various deformities, trauma, or fractures of the vertebra, due to the complexity of the human anatomy, surgeons may attempt to “fuse” the vertebrae by bending the rod (causing notches thereby reducing fatigue resistance) before placing them into two or more non-aligned pedicle screws in order to properly stabilize the pedicle screw assembly within the patient's body.
Furthermore, most conventional screw receiving portions do not generally offer enough medial-lateral flexibility because the rod sits too closely on top of the center of rotation of the bone screw producing a smaller arc of rotation. Moreover, most conventional screw systems do not generally accommodate different rod sizes. Additionally, most conventional spinal implant designs typically lack features that enhance invasive surgery techniques that are used for spinal surgeries which can decrease the patient's recovery time. Also, there is generally a lack of limitation of load sharing ability, which may lead to damage of the vertebrae during natural motion. Thus, there remains a need for a new and improved dynamic pedicle screw assembly with intra-operative flexibility, that allows the bone screw to be tensioned in a given zone or range of angulations in the screw head while permitting natural motion.